The subject matter of the present invention pertains to beam-index color display systems wherein an index signal generated within a beam-index cathode-ray display tube is employed to control the selection of digital information for display by the tube. For a detailed description of such tubes and systems, see, for example, A. M. Morrell et al, "Color Television Picture Tubes," Academic Press, New York, 1974, all pertinent parts of which are incorporated herein by this reference.
In a conventional beam-index display system, the index signal, generated by the electron writing beam as it moves across certain phosphor bands disposed over the faceplate of a beam-index cathode-ray display tube, is employed to control the sequential selection of portions of one or more video drive signals for successive transmission to the beam-producing apparatus of the tube in timed sequence with the movement across the phosphor bands, each drive signal being representative of a differentiable characteristic, usually color, of an image to be produced. Normally, the various phosphor bands are oriented vertically and disposed across the tube faceplate in a horizontal direction, the same direction as the beam movement. Since the index signal is a function of the position of the writing beam relative to that of the phosphor bands, its use to control the selection of the input drive signals ensures to a certain degree that each selected signal will be converted into a respective image component at a predetermined location on the tube faceplate. The beam-index concept is especially useful in the generation of high-resolution color images because of its ability to sequentially select portions of input signals representing various color characteristics of an image and present the selected signals to the display tube at the precise times that the writing beam is in position to impinge upon the corresponding bands of color phosphor.
A disadvantage of prior art beam-index display systems is that the index signal is employed to control only the selection of the input drive signals and not their initial generation. While such single quantization of the input drive signals is usually sufficient in an analog environment, for example, when processing a conventional television-type video signal of relative low resolution where a particular piece of color information may extend on the display screen over several bands of a respective color phosphor, it is less than sufficient in a high-resolution digital environment where a particular piece of color information may be destined for a particular phosphor band and, if not presented there at the precise instant that the writing beam is in position to affect the phosphor, be permanently or intermittantly lost.